Well it depends on your mobile network provider. In my case initially I was using command
nmcli c add type gsm ifname ttyUSB2 con-name 4gnet apn internetipv6 gsm.username internetipv6 gsm.password internetipv6
and when faced connection stability issues I have replaced it with
nmcli c add type gsm ifname ttyUSB2 con-name 4gnet apn internet gsm.username internet gsm.password internet
Now the connection is stable. Speed is not so huge though, but I think it is related to GSM antenna placement.
In summary, probably your network settings are incorrect and you need to fix it. I suppose